Development of the next generation of semiconductor devices has emphasized the use of metals with lower resistivity values, such as copper, than previous generation metals in order to reduce capacitance between conductive layers on the devices and to increase the frequency at which a circuit can operate. One way to fabricate planar copper circuit traces on a silicon dioxide substrate is referred to as the damascene process. In accordance with this process, the silicon dioxide dielectric surface is patterned by a conventional dry etch process to form holes and trenches for vertical and horizontal interconnects. The patterned surface is coated with an adhesion-promoting layer such as tantalum or titanium and/or a diffusion barrier layer such as tantalum nitride or titanium nitride. The adhesion-promoting layer and/or the diffusion barrier layer are then over-coated with a copper layer. Chemical-mechanical polishing is employed to reduce the thickness of the copper over-layer, as well as the thickness of any adhesion-promoting layer and/or diffusion barrier layer, until a planar surface that exposes elevated portions of the silicon dioxide surface is obtained. The vias and trenches remain filled with electrically conductive copper forming the circuit interconnects.
Tantalum and tantalum nitride are particularly suitable materials for use in the damascene process as adhesion-promoting and/or diffusion barrier layers for copper-based devices. However, the properties of tantalum and of tantalum nitride differ from those of copper, being considerably more chemically inert, such that polishing compositions useful for the polishing of copper are often unsuitable for the removal of underlying tantalum and tantalum nitride. Typically, the polishing of tantalum layers requires compositions comprising an oxidizing agent, such as hydrogen peroxide, as well as high solids loading, i.e., >5 wt. % of abrasive based on the total weight of the composition, to achieve useful removal rates. However, hydrogen peroxide is a strong oxidizing agent that can react with other components of polishing compositions, which limits the stability of the polishing compositions and thus their useful pot-life. As such, hydrogen peroxide is usually added to the polishing composition by the end-user, i.e., it is used in so-called “two-pot” compositions entailing additional mixing steps in the polishing operation. Further, strong oxidizing agents such as hydrogen peroxide can erode copper lines on the substrate surface by chemical etching. High solids or abrasive loadings are also problematic. For example, high solids content can produce defects on the surface of the substrate that can negatively impact the performance of any integrated circuit layer manufactured from the substrate and can lower the tantalum to silicon dioxide selectivity. Furthermore, polishing compositions containing a high solids content are more expensive to produce.
Thus, there remains a need for improved polishing compositions and methods of polishing for substrates comprising tantalum and copper that are cost effective and decrease overall process time, such as by increasing the removal rate in tantalum.